Stray-light reducing device for an optical system

ABSTRACT

A device is part of an optical system ( 1 ) that works with an intermediate image and comprises stop( 5 ) with associated aperture ( 6 ). The device comprises a light-reducing or light-absorbing arrangement ( 12 ) located at the side of the aperture. The arrangement is arranged to reduce considerably or eliminate completely light ( 11 ) incident to the side of the aperture, and thereby considerably reduce beam reflections. In this way, there is less likelihood of the optical system being detected when this is in use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Entry of PCT/SE02/01190 filed onJun. 19, 2002, which claims priority under 35 U.S.C. §119 to SwedishApplication 0102332-4, filed on Jun. 29, 2001.

BACKGROUND

This disclosure relates to a device for an optical system that workswith an intermediate image and comprises a stop with an associatedaperture.

In various advanced applications, for example military applications,where sights, telescopes, electro-optical TV cameras, etc are used, theuse of optical systems of this type is already known. The optical systemcan work, for example, within the visual range and the infrared range,that is with wavelengths within the range 0.45-14 micrometres. Inconnection with this, reference is made to the patent literature and theknown systems on the market. By light is meant in this application atleast a wavelength range 0.3-30 μm, comprising visible light and suchwavelengths within the ultraviolet and infrared range that can behandled by optical technology used for the visible range.

With this type of optical system there is a pronounced desire to be ableto design the system in such a way that it does not reveal itself duringuse. The problem is here that there are difficulties in avoiding straylight and reflections that can be detected by detection equipment, forexample laser radar. The invention intends to solve this problem andproposes in this respect that the stray light and reflections inquestion should at least be reduced considerably. In connection withthis, beam-reflecting surfaces (mirror surfaces) are used in oneembodiment, and there is a desire to be able to arrange these surfacesin such a way that an optical system is obtained that is exceptionallydifficult to detect. The invention also intends to solve this problem.

SUMMARY

What can primarily be considered to be characteristic of a device of thetype described by way of introduction is that it comprises alight-reducing and light-absorbing arrangement located to the side ofthe aperture, which arrangement is arranged to reduce considerably oreliminate completely light incident to the side of the aperture, andthereby considerably reduce or eliminate the effects of stray light andreflections.

In one embodiment, the light-reducing or light-absorbing arrangementcomprises a light-reflecting surface arrangement with a surface coatingor surface coatings that give low reflection of the light incident tothe side of the aperture. For this purpose, the surface arrangement cancomprise mirror surfaces set at an angle to each other that results inthe reflection or reflections being obtained in controlled directions.The mirror surfaces in question can be provided with coatings that givea low reflection, the respective reflecting surfaces being arranged sothat a reduction is obtained by, for example, 10-2. In one embodiment,two or more mirror surfaces are arranged so that they produce at leasttwo or three reflections of the light incident to the side of theaperture.

The device that considerably reduces the stray light or reflections canbe designed as a unit that is arranged with an opening extendingcentrally, the mouth of which acts as an aperture. The unit is arrangedwith unique characteristics and reference is hereby made to thesubsidiary claims below.

By means of the device that is proposed above, a technically simplesolution is obtained to the problems mentioned by way of introduction.As the invention utilizes known means and means that do not require theoptical system as such to be redesigned for the implementation of theinvention, economic benefits are also obtained. One and the same systemcan be provided with “light traps” of various designs and performanceand in this way the system can be arranged for a stringent requirementconcerning the elimination of stray light and reflections, orrequirements that it will work with the system as such.

BRIEF DESCRIPTION OF THE DRAWINGS

A currently preferred embodiment of a device that has the significantcharacteristics of the invention will be described in the following withreference to the attached drawings in which

FIG. 1 shows in longitudinal section and in outline an example of anoptical system with intermediate image and where a light trap isarranged in association with an aperture incorporated in the system,

FIG. 2 shows in vertical section a mirror surface arrangement fordealing with the unwanted beams in question and for eliminating theeffects of the same, and

FIG. 3 shows an example of the design of the light trap, viewed inperspective and from above.

DETAILED DESCRIPTION

In FIG. 1, an optical system with so-called intermediate image functionis shown in outline by 1. The system can comprise one or more firstlenses or optical elements 2 and one or more second lenses or opticalelements 3. The system also comprises a detector surface 4. In addition,there is a stop function 5 that comprises an aperture 6. The ordinaryand required beam path is represented by solid lines 7, 8 and 9, 10. Thefunction and the design of this type of optical system is already wellknown and will therefore not be described here in greater detail.

According to the invention, an arrangement is arranged in associationwith the said aperture that at least considerably reduces the beam orthe stray light that goes from the lens or the lens arrangement 2 to theside of the aperture 6. An example of such stray light is represented bya broken line 11 in FIG. 1. The arrangement for the reduction of thestray light 11 incident to the side of the aperture 6 comprises a unitdescribed in greater detail below. It is characteristic of the unit 12that it works with a mirror surface or mirror surfaces 13, 14 which inthe embodiment are coated with a surface coating that considerablyreduces the reflected beam. In the embodiment shown, the beam 11 isincident to a mirror surface 14, which in turn reflects the beam towardsa second mirror surface 13. The said reflected components of the beamare indicated in FIG. 1 by 11 a and 11 b. With the reflection by thefirst mirror surface 14, there is thus an exceptional reduction of thebeam intensity, for example a reduction by 10-2. A correspondingreduction occurs when the beam is reflected by the second mirror surface13, which means that the beam 11 b leaving the unit 12 is considerablyreduced in comparison with the incident beam 11. It is recognized thatthe mirrors 13 and 14 can be arranged at different and suitable anglesof inclination in relation to each other and in relation to the incidentbeam 11. In each individual design, it is thus possible to build in alevel of reduction simply by the use of mirrors and their surfacecoatings, and also to obtain a controlled emission of the reflected beam11 b out of the unit 12. The number of surfaces can also vary and, in apreferred embodiment, a mirror arrangement is used that gives 2, 3 ormore reflections in the V-shaped channel of the unit. The unit 12 isalso provided with a central opening 15, at one end of which, morespecifically the end facing towards the lens 2, the mouth forms theaperture 6.

FIG. 2 shows the construction of an embodiment with reflective surfaces13′, 14′. The device corresponding to the unit 12 according to FIG. 1 ishere divided into a first and a second unit 20, 21. The first unit 20 isin the shape of a pyramid with an opening 15′ that goes right through,which opening is the shape of a truncated pyramid, the sides of whichform angles λ to the longitudinal axis 17. The unit 20 comprises fourouter surfaces that constitute mirror surfaces, 14′. The second unit 21is designed as an internal truncated pyramid, around the longitudinalaxis 17, where the four inner surfaces 13′ are mirror surfaces. In FIG.2 only one surface 13′ and one surface 14′ are shown for the sake ofclarity. The outer part of the first unit 20 and the inner part of thesecond unit 21 meet at the lower part of the device. In this way, aV-shaped channel is produced with reflective edges. The pyramid does notneed to be limited to having four sides. Alternatively, the pyramid canbe replaced by some other shape or by a cone. The reflective surfaces14′ of the unit 20 form the angle α to the longitudinal axis 17. α canbe of the order of 30°. The reflective surfaces 13′ of the unit 21 formthe angle β to the longitudinal axis 17. The angle β can be of the orderof 10°. FIG. 2 shows an alternative incident beam in relation to theembodiment according to FIG. 1. In this case, the incident beam isindicated by a broken line 18. The beam is reflected towards a surfacein question 14′ and continues according to 18 a, is reflected to asurface in question 13′ and continues according to 18 b and is reflectedagain towards the surface 14′ and continues out of the device accordingto 18 c. It is recognized that a considerable reduction can be obtainedin the incident beam by means of the three reflections, twice on surface14′ and once on surface 13′. In the figure, the incoming beam is at anangle γ and the outgoing beam is at an angle δ to the longitudinal axis17. The two angles are different and can be varied in the arrangement.

FIG. 3 shows the first and the second unit 20 and 21 with the mirrorsurfaces 14′ and 13′ respectively. In one embodiment, the embodimentdescribed above as a truncated pyramid is, in principle, interchangeablewith cone shapes for the unit, surfaces, openings, etc. It is alsorecognized that combinations of the two embodiments can be used. It isalso recognized that the surface arrangement in question can be arrangedin such a way that a practically total local elimination of the straylight or the incoming beam 11, 18 is achieved. Respective mirrorsurfaces can be provided with a coating of a known type, for example anabsorbent layer coated with an anti-reflective coating, a so-called“black mirror”.

The invention is not limited to the embodiments described above by wayof example, but can be modified within the framework of the subsequentclaims and concept of the invention.

1. A device for an optical system (1) that works with an intermediateimage and comprises: a stop (5) with an associated aperture (6) throughwhich light passes completely through the device, a light-reducing orlight-absorbing arrangement located along a side of the aperture (6),said arrangement being arranged to reduce considerably or eliminatecompletely light (11, 18) incident to the side of the aperture, andthereby reduce or eliminate stray light and reflections, characterizedin that in the case with the reflection of the light, a surfacearrangement is arranged with its surfaces at an angle to each other thatresults in reflection in a controlled direction or directions,characterized in that the surface arrangement comprises at least twodifferent surfaces or surface arrangements arranged to produce at leasttwo or three reflections of the light (11, 18) incident to the side ofthe aperture (6), characterized in that the light-reflecting orlight-absorbing arrangement comprises a first part (20) that is arrangedwith the said aperture (6) and with one or more external firstlight-reflecting surfaces (14′) an a second part (21) that has one ormore internal second light-reflecting surfaces (13′), and characterizedin that one or more first light-reflecting surfaces are in the shape ofouter cones and one or more second light-reflecting surfaces are in theshape of inner cones, the half cone angles for the surfaces beingunequal.
 2. A device for an optical system (1) that works with anintermediate image and comprises: a stop (5) with an associated aperture(6) through which light passes completely through the device, alight-reducing or light-absorbing arrangement located along a side ofthe aperture (6), said arrangement being arranged to reduce considerablyor eliminate completely light (11, 18) incident to the side of theaperture, and thereby reduce or eliminate stray light and reflections,characterized in that in the case with the reflection of the light, asurface arrangement is arranged with its surfaces at an angle to eachother that results in reflection in a controlled direction ordirections, characterized in that the surface arrangement comprises atleast two different surfaces or surface arrangements arranged to produceat least two or three reflections of the light (11, 18) incident to theside of the aperture (6), characterized in that the light-reflecting orlight-absorbing arrangement comprises a first part (20) that is arrangedwith the said aperture (6) and with one or more external firstlight-reflecting surfaces (14′) and a second part (21) that has a one ormore internal second light-reflecting surfaces (13′), and characterizedin that one or more reflecting first surfaces are located on a unit (20)in the shape of a truncated pyramid, and in that one or more secondlight-reflecting surfaces are located on an outer part (21) of thepyramidal unit, with the second light-reflecting surface(s) beingopposite the first surface(s), the angles of inclination of the firstand second light-reflecting surfaces being unequal.
 3. A stray-lightreducing optical system having a reduced probability of detection by anexternal sensor, the system comprising: an optical lens that provides anintermediate image; a stop having an aperture through which a portion ofthe intermediate image passes; a V-shaped channel located at alight-incident side of the stop adjacent to the aperture, wherein wallsof the V-shaped channel are configured to provide said reducedprobability of detection by an external sensor by reducing stray lightand reflections back through the optical lens, wherein the V-shapedchannel comprises a first part having one or more external firstlight-reflecting surfaces, and a second part having one or more internalsecond light-reflecting surfaces, wherein the one or more external firstlight-reflecting surfaces are located on a unit having a shape of atruncated pyramid, wherein the one or more internal secondlight-reflecting surfaces are located on an outer part of the truncatedpyramid opposed to the one or more external first light-reflectingsurfaces, wherein angles of inclination of the first and secondlight-reflecting surfaces are unequal.
 4. The system of claim 3, whereinthe walls of the V-shaped channel comprise light-reflecting surfaceshaving a coating thereon that provides a relatively low reflection oflight from the intermediate image incident on the light-incident side ofthe aperture.
 5. The system of claim 3, wherein the walls of theV-shaped channel comprise light-absorbing surfaces that absorb lightfrom the intermediate image incident on the light-incident side of theaperture.
 6. The system of claim 3, wherein the walls of the V-shapedchannel are arranged at an angle to each other so as to provide lightreflection in one or more controlled directions.
 7. The system of claim3, wherein the walls of the V-shaped channel are arranged to produce atleast two reflections of the light from the intermediate image incidenton the light-incident side of the aperture.
 8. The system of claim 3,wherein the walls of the V-shaped channel are arranged to produce threereflections of the light from the intermediate image incident on thelight-incident side of the aperture.
 9. A stray-light reducing opticalsystem having a reduced probability of detection by an external sensor,the system comprising: an optical lens that provides an intermediateimage; a stop having an aperture through which a portion of theintermediate image passes; a light-trap around the aperture, wherein thelight trap comprises a first part having one or more external firstlight-reflecting surfaces, and a second part having one or more internalsecond light-reflecting surfaces, wherein said one or more externalfirst light-reflecting surfaces are in the shape of outer cones, andsaid one or more internal second light-reflecting surfaces are in theshape of inner cones, wherein half cone angles for the firstlight-reflecting surface and the second light-reflecting surface areunequal.
 10. A stray-light reducing optical system having a reducedprobability of detection by an external sensor, the system comprising:an optical lens that provides an intermediate image; a stop having anaperture through which a portion of the intermediate image passes; alight-trap around the aperture, wherein the light trap comprises a firstpart having one or more external first light-reflecting surfaces, and asecond part having one or more internal second light-reflectingsurfaces, wherein the one or more external first light-reflectingsurfaces are located on a unit having a shape of a truncated pyramid,wherein the one or more internal second light-reflecting surfaces arelocated on an outer part of the truncated pyramid opposed to the one ormore external first light-reflecting surfaces, wherein angles ofinclination of the first and second light-reflecting surfaces areunequal.
 11. A stray-light reducing optical system having a reducedprobability of detection by an external sensor the system comprising: anoptical lens that provides an intermediate image; a stop having anaperture through which a portion of the intermediate image passes; alight-trap around the aperture, wherein the light trap comprises a firstpart having one or more external first light-reflecting surfaces, and asecond part having one or more internal second light-reflectingsurfaces, wherein an inner part having the one or more external firstlight-reflecting surfaces is arranged with the aperture passingtherethrough, said aperture being configured to widen out towards alower section of the inner part.
 12. The system of claim 11, wherein theaperture comprises an opening having a square cross-section.